Ultra- and microfiltration membranes (UF- and MF membranes) are today widely used in a large number of industries for separation and concentration of solutions and suspensions.
The word membrane is in this context used to describe thin, porous films, foils and sheets, usually in form of permeable flat films, tubes or hollow fibers prepared from various organic types of polymers. UF membranes are characterized by their ability to separate macromolecules from low molecular weight compounds and salts. Membranes are specified by the molecular weight cutoff value, (MWCO). The cutoff value normally refers to well-defined macromolecules (globular proteins) and indicates that macromolecules with a molecular weight higher than the given MWCO are not allowed to pass the membrane, while molecules with a lower molecular weight more or less pass the membrane. MF membranes are characterized by their ability to retain bacteria and other microorganisms, particles, colloids etc. and are specified by the indication of their pore size e.g. determined by the so-called Bubble Point Method as stated in ASTM F316 (Standard Method of Test for Pore Size Characteristics of Membrane for use with Aerospace Fluids).
The expression "hydrophobic membrane" refers in the following to any membrane made more hydrophilic by the method according to the invention.
UF and MF membranes are used in a large number of industries, such as water processing, food, chemistry, pharmaceutics, bio-engineering, waste water processing etc., for various separation and concentration tasks, removal of particles and the like.
In order to be usable the membranes have to meet several requirements, such as
Sufficient physical strength for resisting the mechanical influences the membranes are subjected to during operation in the filtration system in question.
Good chemical resistance, allowing use of the membranes in a wide temperature and pH range in different chemical environments as well as toleration of various detergents and desinfectants.
Good transport properties (flux) and selectivity allowing a given separation/concentration task to be technically and economically feasible.
UF and MF membranes are usually made of highly resistant, synthetic polymers more or less hydrophobic by nature and displaying a great tendency for ad- and absorption of many organic compounds and colloids.
Ad- and absorption results in lower flux values as well as a flux reduction with time (fouling). At the same time the separation properties may be altered, since the ad- and absorbed material on the surface of the membranes forms a layer, said layer being able to act as a secondary membrane.
A number of the above disadvantages are avoided if hydrophilic instead of hydrophobic membranes are used, said hydrophilic membranes being easily wetted by water and known to possess low ad- and absorption of fouling materials (e.g. proteins). They generally possess an advantageously low protein binding tendency and a lesser inclination for fouling, the latter in turn resulting in advantages in form of higher flux values, lesser flux reduction with time as well as easier cleaning and lower cleaning frequency.
Furthermore, it is thus possible to obtain membranes with a lower cutoff value and the membranes may be stored in dry condition, since they are easily rewetted with water.
The British patent specification 1.538.810 describes the hydrophilization of fluoropolymers with water-soluble polymers (i.e. cellulose derivatives and polyvinylalcohol) by rendering said polymers insoluble by means of cross-linking reactions, such as heat treatment, acetalization, esterification, chemical reaction with bichromate, or by means of ionizing radiation. In the present invention, however, the water-soluble, OH-containing polymers are rendered insoluble either by formation of ether bonds between the polymer chains by means of low molecular weight cross-linking agents or by formation of chemical bonds between said polymers and a polyvinylidenefluoride and/or polyvinylidenefluoride copolymer support membrane by means of a base-catalyzed reaction. The above British patent specification does not relate to the use of the hydrophilic, porous fluorocarbon structures, while the present invention improves the properties of the membrane.
The U.S. Pat. No. 3,620,970 describes the preparation of Reverse Osmosis (RO) membranes, i.e. membranes with salt retention properties. The ultrafiltration and microfiltration membranes of the present invention possess a insignificant salt retention (ideally 0% is desired) but an improved membrane surface in order to avoid or reduce ad- and absorption/fouling during hydrophilization.
U.S. Pat. No. 4,340,482 discloses a microporous, polymeric article attaining hydrophilic properties by means of treatment with a highly alkaline solution of an amino acid, preferably glycine. The hydrophilicity is attained by means of grafting with the amino acid being a charged molecule. Membranes with charged groups display pH-related flux and permeability properties during the filtration of many products. Components with opposite charge may thus be bound to the membrane and foul said membrane, while components with the same charge are repulsed and optionally completely or partially retained during the filtration. The use of highly alkaline concentrations and high temperatures in this known method limits its application. Hydroxypropylcellulose being used i.e. in the present invention cannot be dissolved under these conditions.
The synthetic polymers usually used for the preparation of a membrane are distinctly hydrophobic, e.g. polysulfone. Natural polymers, such as cellulose and derivatives thereof, are, on the other hand, hydrophilic and today UF membranes are made of regenerated cellulose. The use of MF membranes made of regenerated cellulose has, however, been problematic because of the ability of this material to swell in water as well as its low mechanical strength due to its high porosity.
Further synthetic polymers include fluoropolymers, homopolymers as well as vinylidene fluorides, such as polyvinylidene fluoride (PVDF) generally less sensitive to fouling than polysulfone. Ultrafiltration membranes made of PVDF have been known for many years and corresponding microfiltration membranes are also produced. One technique includes adding small amounts of a partially mixable fluoro-copolymer (chlorotrifluoroethylene/vinylidene fluoride copolymer (CTFE/VF)) to the membrane solution containing besides PVDF a common solvent, optionally admixed with swelling agents/non-solvents and viscosity-increasing components. The preparation is performed with the traditional machinery in a manner known per se. Another technique makes use of the fluoro-copolymers (CTFE/VF) as sole membrane polymer, by using a high molecular weight, water-soluble polymer as pore former.
In view of the good properties of cellulose it was natural to investigate the possibility of employing the so-called thin film technique for coating polysulfone as well as the above PVDF-based MF membranes. Cellulose is not directly suitable because of its dissolving properties. Thus it is necessary to use derivatives thereof. Hydroxyethylcellulose and hydroxypropylcellulose have been proven to be suitable, since they are water soluble. In order to fixate these materials on the membrane surfaces they have to be made water-insoluble. This is done either by means of cross-linking the polymer chains or by creating a chemical bond to the support membrane. Polysulfone does not immediately present any reaction possibilities, while PVDF as well as the CTFE/VF copolymer present reaction possibilities under highly alkaline conditions and elevated temperatures. Under these conditions hydrogen fluoride/hydrogen chloride are set free while reactive groups and double bonds, the basis of an addition reaction, are formed.
According to the present invention UF and MF membranes with hydrophilic character may be provided on the basis of existing ultrafiltration and microfiltration membranes. The process includes applying an aqueous solution comprising a hydrophilic polymer and possible, reactive monomers as well as a catalyst to the "surface". Then the membrane is heated, e.g. in an oven, to a suitable temperature so that a hydrophilic "surface layer" is formed, said layer being either chemically or physically bound to the membrane material. Thus the properties of the membrane are altered in such a way that the character of the "surface layer" predominantly determines the filtration properties.
In the present specification the term "surface" in connection with membranes refers to any membrane surface being able to come in contact with the liquid to be filtered and thus not only to the outer surface of said membrane.